I live in an older house (40 years old). About three years ago we replaced the stand up shower in our master bath. Yesterday I was under the house checking on things, as I do often since the home is older. I was checking the drain to the new shower. I’ve checked it plenty of times before and never noticed any leaks, stains….nothing out of the ordinary. This time I pulled the insulation back some and noticed a white powdery substance around the outside of the drain connection (pics attached). I had just showered, and it was not wet at all, in fact it was powdery. Almost like effervescence. After my shower today I checked it again, still no wetness. No other stains…nothing. Any idea what this could be?? I talked to the installer, he said he does not use glue on his drains, so it’s not that. The bottom of the shower is fiberglass
